1. Field of the Invention
The present invention relates to a casting process for producing a metal matrix composite having a first phase matrix of metal or metal alloy containing second phase particles dispersed therein.
2. Description of the Related Art
The known metal matrix composites (MMC) are typically composed of a matrix (a first phase or a base material) of a metal or metal alloy and a second phase of reinforcing particles such as ceramic particles dispersed in the matrix. The reinforcing particles or other second phase particles are used in the form of grains, whiskers, fibers, etc. The metal matrix composites having an aluminum or magnesium matrix are particularly excellent because they are lightweight, have a high specific strength, have a high specific stiffness, etc.
Typical processes for producing metal matrix composites include thermal spraying, casting, sintering, plating, etc. The casting process provides high productivity and has already been widely practiced, as summarized in "Kinzoku (Metal)", May 1992, pages 48-55.
In the casting process, of particular importance is the liquid phase process, in which reinforcing particles or other second phase particles are brought into dispersion in a melt of a metal or metal alloy (hereinafter simply referred to as "metal melt", or more simply as "melt") to produce a uniform dispersion of the second phase particles in a matrix of the metal or metal alloy. Typical liquid phase processes include infiltration and eddy current stirring, both requiring special equipment or an adjustment of the alloy composition when using ceramic or other second phase particles having low wettability with a metal melt.
Infiltration requires large scale equipment to apply a high pressure necessary to overcome the low wettability.
Eddy current stirring requires a long time to disperse particles in a metal melt, and moreover, it is very difficult to produce uniform dispersion of fine particles even if stirring is performed for a long time. For example, a parameter indicating the wettability of ceramic particles with an aluminum melt is a balance between a gravity force exerted on the ceramic particles (a sinking force due to the particle volume or mass) and a surface tension (a floating force due to the particle surface area), where the smaller the particle size, the greater the effect of the particle surface area compared to that of the particle volume, so that it becomes difficult to cause fine particles to enter a metal melt.
Thus, uniform dispersion of the second phase particles in a matrix is significantly obstructed by a poor wettability therebetween. Therefore, the conventional processes improved the wettability by coating the particle surface, raising the temperature of the metal melt, or adding Mg, Li, Ca, Sr, Ti, Cu, or other wettability-improving alloying elements to the metal melt.
Another problem of the eddy current stirring is sedimentation and segregation of the second phase particles (reinforcing components) in the matrix metal. For example, ceramic second phase particles mostly have a greater density than an aluminum melt as a matrix metal and sedimentation of the ceramic particles occurs during solidification of the aluminum melt. Moreover, the interfacial energy between a solid aluminum and a ceramic particle is mostly greater than that between a liquid aluminum and the ceramic particle, so that the ceramic particles are segregated at crystal grain boundaries of the solid aluminum matrix.
The occurrence of sedimentation or segregation of the second phase particles in the first phase matrix produces a non-uniform microstructure of a metal matrix composite, which only exhibits a reduced or a strength or other properties varying between portions thereof.
To eliminate these drawbacks, various measures have been taken; crystal grains are refined to apparently reduce the segregation; alloying additives are used to vary the interfacial energy between first and second phases to facilitate incorporation of second phase particles into a first phase or solid matrix; and casting is performed at an increased cooling rate to complete solidification before substantial movement of the second phase particles occurs.